1. Field of the Invention
The present invention relates to a hard disk drive usable as an auxiliary memory for computers, and particularly to an apparatus for fixing in a base a spindle motor rotating disks in the hard disk drive.
2. General Description of Related Art
A term "NRRO(Non Repeatable Run Out)" mentioned hereinafter in the present invention defines a phenomenon caused by non-periodic displacement when a spindle motor revolves about an axis. The NRRO relates to precision in working, shockproofness and stiffness of bearings.
The conventional hard disk drive, as shown in FIG. 1, has a spindle motor 56 that rotates a disk at a predetermined speed and a base 50 in which the spindle motor 56 is fixed. The spindle motor 56 has a hub 58. The hub 58 has screw head mounting recesses 60 and holes 62. The base 50 in which the spindle motor 56 is mounted is a casting. Further, the base 50 has a hub mounting surface 52 and screw holes 54. The hub mounting surface 52 is formed by the precise working of the bottom of the base 50.
As shown in FIG. 2, the hub 58 of the spindle motor 56 is mounted in the hub mounting surface 52 of the base 50 and screws 64 are then driven into the screw hole 54 of the base 50 through the holes 62 of the hub 58 so that the spindle motor 56 can be fixed in the base 50.
Because the precision ratio of working has a limited value, the conventional spindle motor 56, as shown in FIG. 3, is in partial contact with the base 50, so microscopically the contact surface therebetween is not a complete plane, but a plane having areas of rises and depressions. Therefore, the hub of the spindle motor partially contacts the hub mounting surface of the base. Then, portions in which they are not contacted with each other remain as gaps.
Such partial contact changes the stiffness of the fastening part to nonlinearity. That is, the compression force between the vibrating spindle motor and the base changes. Increases in the compression force occur when the contact position increases, and decreases in the compression force occur when the contact position decreases. This means that the compression force in the contacted position affects the stiffness of the fastening part. Such non-linearity disturbs the low frequency vibration of structures and produces relative displacement between the spindle motor and the base.
Therefore, in the hard disk drive, the vibration of the spindle motor has undesired influences on the NRRO characteristic. This brings a track missing error to disk when a head moves to read and write information on the disk. The track missing makes physical errors during an input/output operation to thereby reduce reliability of the hard disk drive.
The inside of the hard disk drive isolated from the external environment normally requires the most high degree of cleanness for error-free operation. Therefore, the inside of the hard disk drive should be prevented from contamination possibly generated by dust or gas. Thus, care must be taken when attaching the spindle motor onto the base to ensure an airtight hard disk drive. In this case, in order to improve the roughness and the flatness of the working surface, much working time is required and more working surfaces makes its production cost high.